After measuring slump, the next step in the fresh concrete field testing workflow is determining wet density, also referred to as unit weight. This test provides a measure of the mass of fresh concrete per unit volume and is used to verify mixture consistency and batch yield.
Wet density testing helps confirm that the concrete delivered to the site aligns with the intended mix proportions. When combined with air content data, this measurement can also be used to calculate yield, relative yield, and cement content — all of which support quality control and documentation in the field.
When performing all of the field tests outlined in ASTM C31, the wet density test can be conveniently tied to step 5: Performing an Air Content Test, because the volume of the air meter bucket (i.e., the measuring bowl) is known. You can easily fill the bucket and weigh it for this density test, and then use that same concrete to determine the air content.
ASTM Reference
ASTM C138
Materials Required
- Rigid, watertight container of known volume
- Calibrated scale (capacity sufficient for filled container)
- Strike-off plate
- Tamping rod (5/8 in. diameter × 24 in. long)
- Rubber mallet
- Scoop or shovel
The measure (container) must be:
- Made of metal or other rigid material
- Watertight and non-absorbent
- Of known and calibrated volume (commonly 0.25 ft³ or 0.5 ft³ depending on aggregate size)
- The bucket of a Type B air meter is ideal
Procedure
Dampen the inside of the container prior to testing to prevent absorption and reduce friction during consolidation.
Determine the tare weight of the empty container and record it. This value will be used to calculate the net weight of the concrete.
Fill the container with concrete in three equal layers, each representing approximately one-third of the total volume.
First Layer
Place the first layer of concrete into the container, distributing it evenly. Rod the layer 25 times using the tamping rod, penetrating through the full depth of the layer without damaging the bottom.
After rodding, tap the sides of the container 10 to 15 times with a rubber mallet to help release trapped air and close surface voids.
Second Layer
Add the second layer and repeat the rodding process, penetrating approximately one inch into the first layer. Tap the sides of the container again after rodding.
Third Layer
Fill the container slightly overfull. Rod the final layer 25 times, penetrating approximately one inch into the underlying layer. Tap the sides of the container again to ensure proper consolidation.
After consolidation, strike off the surface using a strike-off plate with a rolling and screeding motion until the surface is level and flush with the top of the container. Clean any excess concrete from the exterior of the container.
Place the filled container on the scale and record the total weight.
Calculation Overview
The wet density (unit weight) is calculated by subtracting the tare weight of the container from the filled weight and dividing by the known volume of the container.
This value is typically expressed in lb/ft³ (or kg/m³).
When combined with air content results, this test can also be used to determine:
- Yield (actual volume of concrete produced)
- Relative yield (ratio of actual yield to designed yield)
- Cement content (when mix proportions are known)
Forney Fact
This test does more than measure density — it verifies whether the batch delivered is actually what was ordered. A relative yield of 1.00 indicates the correct volume was produced. Values above or below that can signal over-yield or under-yield, both of which can impact performance, cost, and compliance.
With wet density verified and documented, the next step in the field testing workflow is performing an air content test.